For let the world take note,You are the most immediate to our throne;And with no less nobility of loveThan that which dearest father bears his son,Do I impart toward you

This is a shrewd political move by Claudius. By publicly proclaiming Hamlet as the heir, he achieves two things: first, he strengthens and affirms his position as the current King; and second, he throws Hamlet a bone (“Play your cards right, kid, and you may get to be King some day”). As Hamlet’s stepfather, Claudius is setting himself up as Hamlet’s new de facto father…and his sovereign lord.